Wave translation system



Sept. 27, 1938. s BLACK 2,131,365

WAVE TRANSLATION SYSTEM Original Filed March 29, 1933 FIG. I

I 6R/a or FLA TE or LAST rue: M1005, T FIRST TUBE PENTODE OR OTHER TUBE) T l-, 1 F I I Ra M1 \J K I 1; V

I L's Z I R0 I I I \BO I Z L R V I I Kk I I I ,uV 5a KR. N I I I I I I KR, KR? M l W I Mk /l J INPUTT earn/17 cAaLr-cARR/ER BR/ME I B D E c4aLs- CARRIER OR OR OTHER CIRCUIT I I 07115;? t/Rcu/T.

o- :5 1-SR, q

VM- 'WW fl- CIRCUIT NETWORK lNl/ENTOR Hi. 5. BLACK BV/W A TTORNEV Patented Sept. 27, 1938 WAVE TRANSLATION SYSTEM Harold S. Black, Elmhurst, N. Y., assignor' to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 29, 1933, Serial No. 663,317

Renewed February 19, 1936 43 Claims.

This application, is a continuation in part of my copending application Serial No. 606,871, filed April 22, 1932, which issued as Patent 2,102,671, December 21, 1937, for Wave translation system.

This invention relates to wave translation systems, as for instance systems involving wave amplifying means.

An object of the invention is to control transmission properties of such systems, as for example to control impedance relations, modulation, wave reflection, cross-talk, resistance noise, transmission efiiciency, gain frequency relations or singing tendencies involved in the systems.

A feature of the invention relates to effecting such control by feedback of waves in the system. If desired the feedback may be negative feedback or feedback that reduces amplification, such feedback increasing stability of operation and re-. ducing distortion as pointed out in the above 20 mentioned copending application.

In accordance with a feature of the invention as applied for example to a feedback amplifier, the feedback changes the amplifier input or output impedance that faces the sending or receiving circuit, making the controlled impedance approach or matchthe impedance it faces. The feedback may be negative feedback, if desired.

In accordance with a feature of the invention as applied for example to a negative feedback vacuum tube amplifier, the negative feedback action (1) produces substantial change of the amplifier output impedance, making it approach any desired finite value independent of the value of the final stage plate-cathode impedance R or (2) produces substantial change of the amplifier input impedance, making it approach any de- J tionable modulation produced by the amplifier.

sired finite value, independent of the value of the initial stage grid-cathode impedance Ro'.

"I'he desired value for the controlled impedance 40 may be, for example, a value matching or approximating the value of the sending or receiving impedance that faces the controlled impedance.

- The control of the input impedance and the contol of the output impedance, referred to in connection with the above mentioned features, may be eiiected both at thesame time, if desired.

In one specific aspect the invention is a negative feedback vacuum tube amplifier that has an output bridge so connecting the plate-cathode impedance R0 in the tube (or tubes) of the final stage to the load or receiving impedance and to the feedback path that the feedback path would be conjugateto the load if the bridge were balanced, but that hasthe bridge unbalanced. The negative feedback tends to make the amplifier output impedance independent of the impedance value of the R0 arm of the bridge, and changes the amplifier output impedance to render it less different from the value it would have if the bridge were balanced by adjustment of Re. If desired the negative feedback is made large, so that its impedance stabilizing effect is powerful, which causes the amplifier output impedance to approach or approximately equal the value it would have if the bridge were balanced by adjustment of R0, and renders it substantially independent of R0.

If desired, by making the impedance of one of the bridge arms other than the R0 arm so differ from its value for balance as to properly unbalance the bridge, as for example by making the impedance of the arm adjacent the cathode structure sufiiciently greater than its value for balance, the feedback can be caused to increase the amplifier output impedance to a value high compared to the tube impedance R0; so the tube can be worked into an impedance that is high compared to the tube impedance Ro,-and at the same time, and without necessitating material power loss between the tube and the load, the amplifier output impedance and the load or receiving impedance can be matched, for example, by proper adjustment of the impedance ratio of an amplifier output transformer, which, if used, may be regarded as included in the receiving circuit or load. When the tube is, for example, a triode or a coplanar grid tube, the increase of the impedance into which it works can reduce objec- The matching of the amplifier output impedance and the load impedance obviates reflection, and consequent objectionable cross-talk effects in the case, for example, of a cable carrier telephone amplifier.

If desired, the unbalance, of the bridge without feedback, required to make the feedback raise ample to reduce singing tendency. 4

By varying the degree of'unbalance of the bridge without feedback, fiexibility is-readii obtalned in control. of the relation between the amount of the change of amplifier output impedance produced by the feedback and the amount 1 a of negative feedback employed. This is of importance, for example, in facilitating control of the distortion reduction effected by the feedback, which increases with increase of the amount of negative feedback, and in facilitating control of singing tendency, which tendency, in general, increases with increase in the amount of negative feedback employed at frequencies in the utilized frequency range.

If desired, the feedback can be made to lower the output impedance of the amplifier instead of raise it, for instance by making the impedance. of the bridge arm adjacent the cathode structure, or of the opp site arm, lower, instead of'higher, than required to balance the bridge without feed-' back, or for instance by making the bridge arm 1 ance low compared to the tube impedance R0, and

" at the same time the feedback action can be made to match the amplifier output impedance and the;

load impedance, for example, the impedance of the high impedance winding of a step-down amplifier output transformer. This may be desired for instance where the last stage employs the pentode type of tube, in order (without undue transmission loss) to match the amplifier output impedance and the load impedance and yet have pared to R0, for example being matched to the As exoutput bridge impedance which it faces. plalned hereinafter, this can be done without necessitating undue loss in transmission from the final tube of the amplifier to the load and having the impedance of the feedback path low facilitates design of transmission control networksv that may be used in the feedback path and reduces singing tendency of the amplifier.

The negative feedback can be made to correct objectionable variation with frequency that, without feedback, might be produced in the amplifier output impedance by variation of the final stage plate-cathode impedance. For example, the feedback can be made to correct for the poor impedance which, without feedback, triodes and coplanar grid tubes present on their output side at high frequencies, such as the higher of the frequencies of awide frequency range to be transmitted. This high frequency eifect is mainly due to the coupling through the grid-plate capacitance which affects the output impedance of the tube. In this case, the output bridge can be designed as though the plate-cathode impedance were pure resistance, and then, because of the impedance stabilization effected by the negative feedback, the amplifier output impedance at high frequencies (and consequently the low-side impedance presented at high frequencies by an output transformer, if such a transformer is used) f n-saw" the closed feedback loop of the amplifier), for ex becomes much better with feedback than with:

out.'i. e. it approaches, over a wider band, more q nearly an ideal sending impedance, which-isf a pure t resistance; ridei ired, the feedbacki- Z 'effectin'g'thisdmpr'ovement can also change the"- amplifier output impedance throughoutthe uti-L llzed frequency range or match-it to the impedance, orboth, as referred to above."

In one specificv aspect the invention is a'negative feedback vacuum tube. amplifier, which may be'of the type referred to above having the 1mbalanced output bridge, if desired, with an input bridge so..connec ting the grid-cathode impedance R0 of the initial stage to the sending impedance and to the feedback path that the feedback path would be conjugate to the sendingimpedance if the input bridge were balanced, but with theinput bridge unbalanced. Theane'gative impedanceindependent of theimpedance value -of the R0: armuor the bridge. and changes the amplifier input impedance to render it less different from the value it would .have if the bridge were balanced by adjustment of Re. If desired, the negative feedback is made large, so that its impedance stabilizing effect is powerful, which causes theamplifier input impedance to approach or approximately equal the value it would have if the bridge were balanced by adjustment of Ru, and renders it substantially independent of R0.

The amplifiermay be, for example, a cable carrier telephone amplifier with the noise in the outputof the amplifier approaching that due to the resistance noise delivered-to the initial grid, and with-the negative feedback lowering the amplifier input impedance and the impedance attached to the R0 arm of the bridge and matching the amplifier input impedance to the sending impedance in a manner which is somewhat analogous to the manner described above in which negative feedbacklowers the amplifier output impedance and the impedance attached to the R0 arm of the output bridge and matches the amplifieroutput impedance to the receiving impedance, and which is referred to more in detail hereinafter. The matching of the input impedance to the sending impedance reduces wave reflection at the junction of those impedances, and consequent objectionable cross-talk; and, at the same time, since the feedback lowers the input impedance, the value that the input impedance'would have without feedback can be chosen high compared to the sending impedance, to increase the ratio of-signal to resistance noise in the amplifier output, as referred to more in detail hereinafter.

If desired, the feedback changing the amplifier input impedance or output impedance to match the sending impedance or the receiving impedance, respectively, can supplement or replace the impedance transforming action of an amplifier input transformer or output transformer, ram.

feedback tends to render the amplifier input T dering such a transformer unnecessary for impedance transformation or giving flexibility as to choice of an impedance ratio correct for such transformer from the standpoint of impedance matching for example. 7

Other objects and aspects. of the invention will 'be apparent from the following description and claims:

Fig. 1 shows schematically a vacuum tube amplifier circuit embodying a. form of the invention; and

Figs. 2 and a are circuit diagrams facilitating explanation of the invention. Certain. expressions as used herein have the 7B 2,1s1,ses

following significance with reference to vacuum tube amplifiers. Amplification of an amplifier without feedback is the quantity by which the voltage on the grid of the first tube is multiplied to obtain the phase and magnitude of the resulting voltage generated in the plate circuit of the last tube, or the voltage of an equivalent fictitious generator in series with the internal plate resistance of the last tube. willbe designated a (and is a complex quantity). Amplification ratio is the absolute value of the amplification. .Gain is twenty times the logarithm of the amplification ratio.

The complex quantity 1.4.3 will be used herein to designate the ratio by which a voltage of a wave is modified in a single propagation around'the closed feedback loop of a feedback amplifier. It follows that B is the complex quantity by which a driving voltage in the space path of the last tube, in series with the internal plate-filament impedance R0 of that tube, must be multiplied to give the voltage that it-the driving voltage alone-acting through the feedback path, will produce on the grid of the first tube.

As shown in the above mentioned copending application the amplification of a feedback amplifier is -wfi and the correspondingchange in amplification caused by the feedback action is is a quantitative measure of the amount of feedback, and herein, as in that applicatiomthe feedback is described as positive feedback or negative feedback according as the absolute value of is greater or less than unity.

As pointed out in that application, when s l the amplification with feedback approaches which is largely independent of the amplification or variations in amplification of the tubes, and consequently the amplifier gain is stabilized; and, as also pointed out therein, the negative feedback then reduces modulation produced by the amplifier in'substantially the same proportion that it reduces the gain.

For any given frequency, the impedance of a network between any" two points is considered theratio that a voltage applied across the points from an external network would bear to the inter-network current.

The amplifier of Fig. 1 comprises one or more stages of vacuum tubes, as for example, three stages. It has a feedback path shown as a 13- circuit network I, has an output bridge 2 connecting the final stage to the feedback path I and the amplifier output transformer T, and has an input bridge 3 connecting the first stage to the amplifierinput transformer T' and the feedbackpath.

The input and output transformers may connect the amplifier in a cable carrier telephone or other circuit, the amplifier then being used for example for simultaneously amplifying a plu- The quantity This amplification rality of telephone messages, all transmitted together over the cable carrier circuit from multiplex carrier transmitting apparatus (not-shown) Transformer T maybe considered part of the receiving circuit or load. The load or receiving impedance is designated L. Transformer T' may be considered part of the sending circuit. The

sending impedance is designated L.

The amplifier may be of the general type of those shown for example in-Figs. 5, 56 and 57 of the above mentioned copending application, which have a feedback path connected between output and input bridges. The feedback may be negative feedback, and ,ufl may be large; for example, ];43] may be of the order of 50 or 100 in .the utilized frequency range, as in the case of the negative feedback amplifier of the Fig. 57 just mentioned.

As indicated above, the feedback path may, if desired, contain a transmission control network I. This fl-circuit network, if used, corresponds to the transmission control network in the feedback path in the Fig. 5 just mentioned. For ex- .ample, the network I may be an attenuation equalizing network (such, for instance, as the attenuation equalizing, B-circuit networks shown.

in the feedback path of the negative feedback amplifiers of Figs. 56 and 65 of the above mentioned copending application). As pointed out in that application, the equalizer network, when in .the feedback path, contributes transmission characteristics to the overall amplifier circuit that are the inverse of those of the network, and so, by making the attenuation-frequency characteristic of the network similar to that of the cable or circuit to be equalized, the amplifier can be made to equalize transmission over the circuit in which it is connected.

When feedback takes place through an input bridge such as 3, the amplifier input impedance Z' is modified by the feedback only if the bridge is unbalanced. The magnitude and direction of the change in Z then depend on the manner in which the bridge is unbalanced and upon the amount of feedback. Similar conditions hold with reference to the amplifier output impedance Z for unbalances in the output bridge. Formulae are derived below'for Z and Z for variations in each of the arms of the input and output bridges.

Equations will be developed for the impedances when any single arm of the bridges is varied. It can readily be seen that these can be made to cover any case of variation of any or all arms, for variation of any one arm can always be made to rebalance a bridge no matter what the other arms may 'be{ The variation will be represented as a multiplier (L l-A) applied to the original value. For example R0 changes to (1+A)Ro=Ro+ARo. The arms of the bridge and their impedances for a balanced condition of the bridge, are represented by Re, KRn, R and KR for the output bridge and by Re, K'Ro', R and KR for the input bridge. It will result insome simplification if we represent R by m2 and R by The impedance of the a network looking from the output bridge will be SR0 and looking from the input bridge it will be SRo'. Any of the symbols R0, K, S, N,Ro', K, S, N and A may have any realizable valueeither real or complex. Z' and Z E a plifie output i pe a u d t e specified resulting expression as much as possible. conditions and Z and Z0 will be used for the For the balanced bri ze i impedancesin the reference condition or balanced bridges. That is, the condition 01' balanced E =S" o 5 bridges willbeusedasthe reference conditionand I a 4 l Z0 and Z0 will represent the input and output impedances respectively in the reference condi- R, on. w I nzl-w- K R As indicated above, the symbol p will be used R to represent the complex transmission ratio for a x= =R' 1 signal making one trip around the closed loop.

The value of pthat will be used in these rormuy= o v lae is the value of p that existed in the reference I I I' I I I I condition and not the value which may exist'aiter '=I#l3 Y 5 a change is made .in one of the bridge arms. For v 1 instance, if the input impedance is to be computed and a for a. certain unbalance in the 'input bridge we 'z 'aml need to know the value of .up which would exist if the input bridge were balanced and the amplifier and is independent of p.

were otherwise the}, same. Thisis a morefconr 4 2 venient value to use because it is independent of 'm in am the impedance from which, or into which the If the R0 arm changes to Ro(l+4) the imped amplifier is working. ance changes as shown I (1+KI)(1 FB 'I4BA).+ A+ (1, 2"

I I I I 2 7 I KS +s +K K! I I (1+N+K,)(I'-IIB FBA)+A I I KS+S+K I m r I N I 1 K 1- A *MW 2EI=R0I B "B +NIKISI'+NISI+NIKI+I I 1 NIX! I I (NIK,)2s,A I I I I v 6. B NIKIQSI+NISI+NI'K +1 ZI ZOI= R9, I'- Y A v 1+N'K N'K' S'A 1 NK 1 A 4o #fi NIKISI+NISI+NIKI+1 Ifjl-fi is very large In order to find the input bridge impedance a R general five element circuit of a configuration 0 4 similar to an input bridge and with a generator in v the g arm equal to P'z'zy is considered. The cirnations in R m y 45 cult is shown in Fig 2, the elements being desig- If the arm n es f om nated as indicated in that flgure.- R t (1 +A)R l a v N K NK' g 1 K A 1- Z Ro m 50 1 I N'KSA FB +A N K/SI+NISI+NIKI+I I v I "B ZI.. ZI=& ROI 1 8 'N'K')A 55 1+ K NKSA 1 N a B NIKISI+NISI+NIKI+ The mesh equations are:

. 1 N K Solving simultaneously we obtain the value 01' To obtain the input impedance in any case it is only necessary to substitute for g, t, n, :c, and Z R 'h P' the corresponding values in terms of bridge 1+N'K arms and transmission ratio and simplify the 75 using, in-development of the impedance formulae, the value of #5 which existed in the balanced case. 7

Output bridge If m8 is very large O'(1+K'+K' 1+NK+.A

1 I I R A( K ,)NK Z It will be noticed that feedback tends to make the input impedance independent of the value of the R0 arm. Thus variations of this arm of the bridge have very little effect onthe amplifier input impedance.

feedback (i. e., with l p] large) is the value which Variations in the other arms do produce an effect and the value of am-f plifier input impedance approached with large- Where for the b alanced case Variations in"R0 a1m If the Ru arm changes to Ro(1+A) K NA j I B)+A+m N K SA U #B +NKA NS +1 NK R0 A 1+-N N K SA I V (H' f NKs+Ns+1-|- NK] would exist if the bridge were rebalanced by a f m s large corresponding variation in the R0 arm.

o( It will also be noticed that the value of the Z n= input impedance is not dependent upon the impedance from which the amplifier works. The value of #18 actually existing is however dependent upon this impedance in any case of imperfect bridge balance and hence the desirability of 1+NK Variations in the R amt If the R arm of the bridge changes from 1144B is 18118 Variations in KR; arm

If the KRo arm changes to'KRo(l+A) NKA the value of up eflective in determining the input impedance depends on the load worked into and similarly the value of p eil'ective in determining the output impedance depends on the impedance from which the amplifier works. Unless the bridges are very much unbalanced the eiiect will not be nearly so prominent.

As noted above, the value for change in output impedance'due to variation in the R0 arm is NKS-i- NS+ 1+ NK 1+1v1 moo-pa) KN'SA Z (1+ 1+.NK 1+NK+NKA Variations in KR am If the KB arm changes from D I F to -i' If AB is large Z LR,(1+K+KA) 1+NK+A R,A(NK -1) The equation for the input'impedance when the R; arm varies is very similar to that of the output impedance when the R0 arm varies. In the case of the other three arms the corresponding equations take ekaetly the same form for the input and the output impedances.

The output impedance is practically independent of the value of the R0 arm for large values of feedback (1. e., with l p] so large that unity can be neglected in comparison to lpfll) and is entirely independent of the impedance of the load into which the amplifier works. Variations in the other arms do produce an efiect, and the value of amplifier output impedance approached with large feedback is the value which would exist if the bridge were rebalanced by varying the Re If both bridges have some unbalance, then (especially if the feedback path contains no network such as l) the input impedance Z will depend to some extent upon the load impedance and the output impedance will depend to an extent upon the impedance fromwhich the amphfler works. This is similar to the case of amplifiers having no conjugacy of the feedback path and line impedances. The reason for this is that NKS-l- NS+ 1 +NK From this equation it can be seen that if is small AR. -#fi Thus, for large values or feedback, where the quantity p is large, the change in output impedance due to ARO may become negligible, as noted above.

In accordance'with the invention, this feature can be used to correct the output impedance when a tube is used which, in the absence of feedback, does not cause the amplifier output impedance to match the impedance of the output transformer. The procedure is to design the output bridge as if the tube had a value of R that,

without feedback, would produce the match; for then, with large values of feedback, the feedback action will subtsantially produce the match, since the change in amplifier output impedance produced by ARo will approximate ARo divided by One illustration of practical application of this 'featureoccurswhere the output tube of a circuit such for example as that of Fig. 1 is a pentode and it is desired to work the pentode into its output impedance, as is customary for pentodes. To do this, an output transformer is used having a high side impedance of approximately Vs the tube impedance, (assuming the case in which K and NK are small as is usually desirable for rected without such networks or loss; for by reducing KRQ to v the value-required to balance the bridge, the low side impedance is corrected although the tube still works into /5 its output impedance.

In cases for instance where it is desired to maintain a close match between the load impedance and the amplifier output impedance and at the same time, for example, obtain the advantages of a pentode, the invention has the further advantage that the impedance out of which the s-circuit works can be materially reduced over the value that it would have without the invention, assuming that in the two cases the transmission loss from the plate circuit generator to the load and into the p-circuit is unaltered.

The ability to reduce this impedance without changing the p-circuit loss very often facilitates the design of c-circuit networks.

For instance, the feedback path may contain an attenuation equalizing or other transmission control network I as described above; and then decreasing the impedance that SR0 faces will permit reduction of SR0 for a given transmission loss from the last plate generator through the p-circuit. This facilitates reduction of the inductances and increase in the capacitances of the network. This diminishes the difllculties due to inherent capacities of induction coils and to inherent wiring capacitance appreciable in com- .parison with the capacities of associated condensers. Moreover, reducing SR0 is important as reducing singing tendency of the amplifier caused by the capacities-to-ground of the network condensers and coils acting as capacities across the feedback path, the effect of these shunting capacities in producing the singing tendency being greater the higher the value of SR0.

Another practical application is found in correcting for the poor impedance which three element and coplanar grid tubes present at higher frequencies. As indicated above, this high frequency efiect is mainly due to coupling'through the grid plate capacitance which affects the output impedance of the tube. The tube may be, for example a triode or coplanar grid tube in the last stage of an amplifier such as that of Fig. 1. In this case the bridge 2 is designed as if the output impedance R0 of the tube were a perfect resistance, the result being that, at high frequencies the low side impedance presented by the output transformer is much better with feedback than. without.

Another application pointed out by way of example occurs where, for instance in a vacuum tube amplifier such as that of Fig. 1 with the last stage using a triode or coplanar grid tube, objectionable modulation voltages generated in the last tube are made small by making the receiving impedance L large compared to R0, and the bridge is unbalanced, for instance by increasing KRo, to make the negative feedback raise the output impedance Z to a value approaching or matching the large receiving impedance L. Matching the impedances andmaking themodulation small may be desired for example in order toreduce objectionable cross-talk effects, for instance where the amplifier amplifies simultaneously a plurality of carrier telephone messages transmitted over a multiplex cable carrier or other circuit in which the amplifier is connected.

From these illustrative applications it is clear that important advantages canbe obtained. by purposely unbalancing the output bridge 2. For example, as just indicated, by having KRo properly depart from the value for balance, a

triode or other suitable tube can be worked into an impedance high compared to its own impedance R0 to reduce modulation and at the same time, and without undue transmission loss, the impedance that presents itself by the secondary side of the output transformer to the connecting cable or circuit can be made any specified value, frequently a matched impedance being sought. Likewise, as just indicated, by having KRo properly depart from its value for balance, a tube such as a pentode can be worked into an impedance very low compared to its own impedance R0 and a similar result obtained. Further, where it is desired to use a transformer .whose ratio, however, on the basis of a balanced bridge, would be wrong, the impedance relations can be corrected by unbalancing the bridge 2, for example by halving KRo properly depart from the balance va ue.

Like results can also be obtained by properly unbalancing the R or KR arms of bridge 2, and as indicated above, an advantage is that the same desired impedance correction will be obtained irrespective of the arm manipulated whereas the efiect of unbalancing the bridge 2 upon the phase shift around the p-path might be harmful if one of these arms KRo, KR and R is varied although it can be made beneficial, for instance by reducing singing tendency of the amplifier, if another of these three arms is varied instead.

These applications of unbalancing the output bridge illustrate an important feature of the invention that, where the output impedance without feedback as presented by an amplifier is desirable for important transmission reasons, but is wrong for impedance reasons (for example, impedance matching), then by feedback the latter aspect is corrected, and, if desired, without appreciably affecting the former.

The input bridge 3 has properties similar to those of the output bridge 2, and in accordance with the invention advantageis taken of the effects of unbalance of the input bridge, as in the case of the output bridge.

To illustrate, in cable carrier and at the higher frequencies the cable noise approaches thermal agitation, and if the voltage step-up of the input transformer T is made sufficiently high tocause the resistance noise and the signal to override the noise introduced by the tubes, for example, of the amplifier, then the noise at the output of the amplifier will tend to approach this thermal noise at the input magnified by the gain of the amplifier. This is the case for matched sending and input impedances, i. e. for L=Z'. As indicated for example .-by United States patent to H. W. Dudley, No. 1,836,841, dated December 15, 1931, if the input impedance is high, instead of a match for the attached sending impedance, the amplifier gain goes up 6 decibels but the thermal noise at the output will be increased only 3 decibels, so there is a net improvement in signal to-noise ratio of 3 decibels. It is also to be noted that, for all configurations of negative feedback amplifiers, the thermal noise at the output is reduced by the amount of feedback or in other words is, as without feedback, the noise at the input amplified by the gain of the amplifier.

Consequently, in accordance with the invention the impedance Z is made high without feedback; and then, because one of the four arms of the input bridge 3 is adjusted to properly unbalance the bridge, with feedback the impedance Z matches the impedance L of the input con- (ill necting circuit. Curiously enough, while the impedance is thus matched as with a balanced bridge having its arms of proper impedance values, the signal-to-noise ratio at the output is improved 3 decibels. A decibel obtained in this way is as highly useful as a decibel increase in level.

What is claimed is:

- 1'. A wave translating system, a circuit associated therewith and a path producing negative feedback therein in the utilized frequency range, said system comprising means connected between said circuit and said path and causing said path to approach but not attain conjugacy with respect to said circuit, the departure from conjugacy being adjusted to control the impedance of said system facing said circuit.

2. A wave translating system, means supplying waves thereto and a path producing negative feedback therein, said system comprising a network connected between said means and said path and causing said path to approach but not attain conjugacy with respect to said means, the departure from conjugacy being adjusted to control the impedance of said system facing said means.

3. A 'wave amplifying system having an input and an output, a wave source having impedance coupled to said input, and a path feeding back waves from said output to said input to control being adjusted to control the input impedance of said system facing-said source.

4. A wave translating system, a load therefor and a path producing negative feedback therein, said system comprising means connected between said load and said path and causing said pathto approach but not attain conjugacy with respect to said load at a utilized frequency, the departure from conjugacy being adjusted to control the impedance facing said load.

5. A wave amplifying system having an input and an output, a load having impedance coupled to said output, and a path feeding back waves from said output to said input to control the operation of said system, said system comprising a bridge network connected between said load and said pathand causing said path to approach but not attain conjugacy with respect to said load and including said output in one of its ratio arms, the denature from conjugacy being adjusted to control the output impedance of said system facing said load.

6-. A wave translating system comprising a wave amplifier with a sending circuit supplying waves thereto and a receiving circuit receiving waves therefrom, and with means producing in said amplifier feedback that causes substantial change -in its input impedance, the impedance of said sending circuit substantially matching the changed input impedance of the amplifier.

7. A wave translating system comprising a wave amplifier with a sending circuit supplying waves 8. A wave translating system comprising wave translating means, means producing negative feedback in said translating means, and a circuit connected to said translating means and having an impedance of value that is nearer to the value of the impedance which it faces in said translating means than would be the case in the absence of the feedback in the translating means.

9. A wave translatingsystem comprising wave amplifying means with its input impedance substantially changed by negative feedback, and a circuit connected to said input impedance and having impedance substantially matching said changed input impedance.

10. Awave translating system comprising wave amplifying means with its output impedance substantially changed by negative feedback, and a circuit connected to said output impedance and having impedance substantially matching said changed output impedance.

11. The method of operating awave translate and its associated sending impedance which comprises producing substantial change in the translator input impedance by negative feedback of waves in the translator, and matching the associated sending impedance to the changed input impedance.

12. The method of operating a wave translator and its associated receiving impedance which comprises producing substantial change in the translator output impedance by negative-feedback of waves in the translator, and matching the associated receiving impedance to the changed output impedance.

13. A wave translating system comprising a source of waves, a vacuum tube amplifier having a grid connected to said source for amplifying said waves, and means producing negative feedback to said grid of waves from said source am-- plified by said amplifier, the impedance of said sourcematching the amplifier input impedance that it faces. I

14. A wave translating. system comprising an amplifier, a wave source attached to the input impedance of said amplifier, and means for substantially reducing the amplifier input impedance and the difference between that impedance and the attached impedance of said source, said means comprising means producing negative feedback in said amplifier.

15. A wave translating system comprising an amplifier, a multiplex carrier telephone circuit attached to the input impedance of said amplifier and so shielded from extraneous noise that its resitsance noise is the principal noise it delivers to said amplifier, said amplifier comprising means so amplifying waves from said circuit that the principal noise in the amplifier output is said resistance noise amplified, and means for substantially reducing the amplifier input impedance and the difference between that impedance and the attached impedance of said circuit, whereby for a given approach to matching of the amplifier input impedance and the attached impedance of said circuit, in the amplifier output the ratio of signal to resistance noise is increased, said last means comprising means producing negative feedback in said amplifier.

16.. A wave amplifying system having a load impedance of value that causes objectionable modulation produced in the system to be less than if the load impedance had the value that the output impedance of the system would have in the absence of feedback, and means for producing in saidsystem negative feedback that matches the output impedance of the system to saidfirst mentioned load impedance.

17. A wave translating system comprising a multiplex carrier signaling wave amplifier having a load impedance of value that causes objectionable modulation produced in the amplifier to be less than if the load impedance had the value that the amplifier output impedance would have in the absence of feedback in the amplifier, and means for producing in the amplifier negative feedback that increases the output impedance of the amplifier and matches it to said first mentioned load impedance.

18. A wave translating system comprising an amplifier having a pentode valve in its last stage, and a load therefor having an impedance that causes objectionable modulation produced in thesystem to be less than if the load impedance had the value that the output impedance of the valve would have in the absence of feedback, and means for producing in said system negative feedback that matches the output impedance of the valve to said first mentioned load impedance.

19. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying element in one of its balancing arms, said bridge being unbalanced, for controlling transmission properties of the amplifier.

20. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a sending impedance attached to the input impedance of the amplifier, and an amplifier input bridge having said feedback path and said sending impedance in its diagonals, respectively, and having the input of said amplifying element in one of its balancing arms, said bridge being unbalanced, for controlling transmission properties of the amplifier.

21. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying element in one of its balancing arms, said bridge being so unbalanced that if said one arm had the value required to balance the bridge the amplifier output impedance without feedback would match the load impedance facing the amplifier output impedance.

22. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier input impedance without feedback would match the sending impedance facing the amplifier input impedance.

23. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying element in one of the balancing arms of the bridge, one of the remaining balancing arms so departing from the balancing value that a prescribed value of amplifier output impedance is obtained which is substantially different from the value that would be obtained without feedback, and which approaches the value that would be obtained without feedback if the bridge were balanced by adjusting'the first mentioned balancing arm.

24. An amplifier comprising an amplifying element, a feedback path for producingnegative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying elementin one of the balancing arms of the bridge, the opposite balancing arm so departing from the balancing value that a prescribed value of amplifier output impedance is obtained which is substantially different from the value that would be obtained without feedback, and which approaches the value that would be obtained without feedback if the bridge were balanced by adjusting the first mentioned balancing arm.

25. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying element in one of the balancing arms of the bridge, an adjacent balancing arm so departing from the balancing value that a prescribed value of amplifier output impedance is obtained which is substantially different from the value that would be obtained without feedback, and which approaches the value that would be obtained without feedback if the bridge were balanced by adjusting the first mentioned balancmg arm.

26. An amplifier comprising an amplifying element, a feedback path for producing negative feedback in the amplifier for stabilizing operation of the amplifier and reducing modulation in the amplifier, a load attached to the amplifier, and

an amplifier output bridge having said feedback path and said load in its diagonals, respectively, and having the output of said amplifying element in one of the balancing arms of the bridge, the remaining balancing arms of the bridge having values such that the output impedance of the amplifying element, the output impedance of the amplifier and the load impedance each has approximately the same prescribed value, this prescribed value substantially differing from the value of amplifier output impedance that would be obtained without feedback and approaching the value of amplifier output impedance that would be obtained without feedback if the bridge were balanced by adjusting the first mentioned balancing arm.

amplifier, a load therefor, and means for lowering the value of the amplifier output impedance and making it nearer the value of the load-impedance, said means comprising a feedback path for the amplifier containing a transmission control network.

28. In combination. a wave translating system, a circuit associated therewith, and means producing in said system two components of negative feedback whose effects upon the magnitude of the impedance of said system facing said circuit are each substantial and are opposite in sense and substantially different in amount, whereby the relation between said impedance and the resultant amount of feedback may be controlled. l

29. The method of operating a signal wave translating device which comprises reducing wave reflection at the input of said device without reducing the ratio of -signal to resistance noise in the output of the device.

'30. A signal wave translating device having input impedance high compared to its attached sending impedance, and means for feeding back in said device waves that reduce reflection at the jlinction of said impedances without reducing the ratio of signal to resistance noise in the output of the device.

31. A wave translating system comprising an amplifier, a circuit attached to the input impedance of said amplifier, and means for substantially reducing the amplifier input impedance and the difierence between that impedance and the attached impedance of said circuit, whereby for a given approach to matching of the amplifier input impedance and the attached impedance of said circuit, in the amplifier output the ratio of signal to resistance, noise is increased, said last means comprising means producing negative feed-back insaid amplifier.

32. A wave translating circuit having its input impedance high compared to the sending impedance which faces the input impedance, and means feeding-back in said circuit waves that reduce reflection at the junction ofthose impedances.

33. A wave translating system having its input impedance substantially different from the sending impedance which faces the input impedance, and means for reducing said difference, said means comprising a circuit for feeding waves back in said system in gain-reducing phase and with amplitude sufilcient to produce substantial alteration of said input impedance.

34. A bridge network comprising an amplifying device, a feedback path for producing negative feedback in said device, a circuit for connection to said device, and interconnecting means for connecting said device to said circuit and to said feedback path, said interconnecting means including an impedance element for imperfectly balancing the impedance of said device in said bridge network in the absence of said feedback, the impedance without feedback viewed from said circuit in the direction of said interconnecting means being dependent upon the impedance of said path viewed from said interconnecting means, and said feedback reducing the effect of changes of said second mentioned impedance upon said first mentioned impedance.

35. A wave translating system comprising an amplifying device, a feedback path for producing negative feedback in said device in the used fre-' quency range, a circuit for connection to said device, and a bridge network connecting said greases device to said circuit and to said path. said bridge network being unbalanced in the absence of said feedback, and the voltage amplification ratio for propagation once around the feedback loop having a value of a higher order of magnitude than unity.

36. A wave translating system comprising an amplifying device, an incoming circuit for connection thereto, and means for feeding back in 37. A thermionic valve amplifier having anoutput stage which comprises a thermionic valve of the pentode type, wherein there is provided a negative feedback circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent upon the voltage set up across the load impedance and reducing the amplifier output impedance below its value with zero feedback from said output circuit to said input circuit.

38. A thermionic valve amplifier having an output stage comprising a thermionic valve of the pentode type, wherein there is provided a negative feedback circuit which is connected and arranged to feed backifromthe output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and wherein the arrangement is such that, if it were used with a load which, by virtue of the feedback, were matched, the magnitude of the feedback voltage would be dependent more upon the voltage set up across the load than upon the current in the load.

39. A thermionic valve amplifier having an output stage comprising a. thermionic valve of the triode type, wherein there is provided a negative feedback circuit which is connected and arranged to feed back from the output circuit of said stage to the input circuit thereof a voltage dependent both upon the voltage set up across the load and upon the current in the load, and wherein the arrangement is such that, if it were used with a load which, by virtue of the feedback, were matched, the magnitude of the feedback voltage wouldbe dependent more upon the current in the of said valve is matched to the load impedance by means of one or more transformers of suitable ratios.

42. A wave translating system comprising a pentode space discharge device having an input circuit and an output circuit,'a load circuit connected to said output circuit and a path feeding back waves in the frequency range of the waves to be translated and of magnitude dependent on the voltage across said load circuit from said output circuit to said input circuit in such phase and magnitude as to reduce the gain of the system and lower the output impedance of said device below its value with zero feedback from said output circuit to said input circuit.

43. A wave translating system comprising a pentode space discharge device having an input circuit and an output transformer, an impedance through which the alternating plate current of said device flows connected in series with said output transformer, means deriving waves from said impedance and feeding them back to said input circuit in such phase and magnitude as to produce negative feedback that tends to raise the output impedance of said device and reduces distortion that would arise in said device in the absence of feedback in said device, and means producing negative feedback in said device dependent in magnitude upon voltage across said transformer and having greater eflect than said first-mentioned feedback on the output impedance of said device.

EAROID S. BLACK. 

